This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Patent Application 2000-361368 filed on Nov. 28, 2000, the entire content of which Is incorporated herein by reference.
This invention generally relates to a control system for an automatic transmission. More particularly, this invention pertains to a neutral control and a hill-hold control performed by a control system for an automatic transmission.
Conventional automatic transmissions are controlled to establish a neutral condition when a driving range has been selected and a vehicle is under a stopped condition. This is generally called a neutral control. However, these types of automatic transmissions suffer from certain disadvantages in that when the neutral control is performed in the transmission, no vehicle-driving power is generated. Further, a vehicle located on an up-hill or incline may move backward.
To address these drawbacks, recent developments have led to an automatic transmission provided with a mechanism for preventing the output shaft from being rotated in a reverse direction. The automatic transmission performs the neutral control while a hill-hold control is performed. This type of automatic transmission is disclosed in Japanese Patent Application Toku-Kai-Hei 11-166614. The disclosed automatic transmission is provided with a hydraulic power transmission connected to an engine, a clutch, a brake, a stop state detector, a clutch disengaging mechanism, and a brake engaging device. The clutch is engaged when a forward driving range is selected. The brake is engaged to prevent the vehicle from moving backward and to thereby establish the hill-hold state in the transmission. The stop state detector detects xe2x80x9ca vehicle stopped statexe2x80x9d when the accelerator pedal is released and the brake pedal is depressed. The clutch disengaging mechanism performs the neutral control upon the clutch being almost disengaged when the vehicle-stopped state is detected by the stop state detector. The brake engaging mechanism performs the hill-hold control upon the brake being engaged when the clutch is almost disengaged by the clutch disengaging mechanism.
With the above-disclosed automatic transmission provided with the aforementioned features, the neutral control is performed without a load on the engine. Improvements in fuel consumption may be realized and the transmission of engine vibration to the driver""s seat may be prevented.
However, the above-disclosed automatic transmission may still suffer from certain drawbacks. For example, the hill-hold control is performed without regard to the vehicle weight. When the hill-hold control is employed for a vehicle, for example a commercial vehicle, in which the load may vary to a relatively large extent, sufficient brake engaging pressure may not be generated in the event the load is relatively large. Therefore, it may not be possible to perform a sufficient hill-hold control.
Of course, the brake engaging pressure may be set to be large enough to perform the hill-hold control for the situation when the load of the vehicle is relatively large. However, when the vehicle provided with a large set brake engaging pressure is not accelerated on a steep up-hill with a relatively small vehicle load, the vehicle is not retracted backward or does not roll backward. Therefore, a driver may feel uncomfortable driving the vehicle.
Accordingly, the disclosed automatic transmission is susceptible of certain improvements with respect to performing an appropriate hill-hold control corresponding to variations in the vehicle weight while the neutral control is performed.
A control system for an automatic transmission includes a torque converter, a first friction engagement element, a second friction engagement element, and a third friction engagement element. The torque converter is disposed between a driving power source and a speed change gear unit, and is employed for hydraulically transmitting rotation of the driving power source to the speed change gear unit. The first friction engagement element is engageable to connect the torque converter and the speed change gear unit, while the second friction engagement element is engageable to perform a forward driving shift stage in the speed change gear unit. The third friction engagement element is engageable to generate a force to prevent an output shaft of the driving power source from being rotated in a reverse direction, and to change the force in response to an engaging force of the third friction engagement element. The control system for the automatic transmission further includes a vehicle stopping detecting means, a vehicle weight detecting means, and a control means. The vehicle stopping detecting means detects a vehicle stopped condition when the forward driving range is selected, and the vehicle weight detecting means detects the vehicle weight. The control means controls the disengaging operation of the first and second friction engagement elements, and the engaging operation of the third friction engagement element by the engaging force in response to the vehicle weight detected by the vehicle weight detecting means when the vehicle stopped condition is detected by the vehicle stopping detecting means while the forward driving range is selected.
Accordingly, when a neutral control is performed with the first and second friction engagement elements disengaged while the forward driving range has been selected, a sliding loss in the torque converter is decreased. Further, in response to a vehicle weight, a hill-hold control is performed to change the force to prevent the output shaft of the driving power source from being rotated in the reverse direction.
The first and second friction engagement elements may be formed by a single friction engagement element so that the number of elements to be controlled upon the neutral control is preferably decreased.
Another aspect of the invention involves a method of controlling a vehicle automatic transmission that includes a torque converter which hydraulically transmits rotation of a driving power source having an output shaft to a speed change gear unit, a first friction engagement element engageable to connect the torque converter and the speed change gear unit, a second friction engagement element engageable to perform a forward driving shift stage in the speed change gear unit, and a third friction engagement element engageable to generate an engaging force to prevent the output shaft from being rotated in a reverse direction. The method includes determining a weight of the vehicle, determining a stopped condition of the vehicle when a forward driving range is selected, and controlling a disengaging operation of the first and second friction engagement elements and an engaging operation of the third friction engagement element by the engaging force based on the determined weight of the vehicle when the vehicle is determined to be stopped while the forward driving range is selected.